Materials used to construct various types of chemical plants need to have excellent corrosion resistance in addition to a sufficient strength. The specific type of corrosion resistance which is required varies with the type of plant, with acid resistance sometimes being required and alkali resistance at other times being required.
As an example of alkali resistance, materials used to construct electrolytic soda plants need to have resistance to a high-temperature concentrated alkali environment.
Examples of materials having such resistance are pure Ti, Ti alloys, and pure Ni. However, each of these is an expensive metal, and they are not practical for use in large-scale plants. Therefore, it is common to use stainless steel, which is relatively inexpensive. However, corrosion resistance of stainless steel is not sufficient compared to that of the above-described metals. Therefore, operation of this type of plant entails frequent replacement of plant components made of stainless steel. Because this replacement is accompanied by a decrease in productivity and an increase in product costs, there is a demand for a stainless steel having excellent corrosion resistance.
A stainless steel which can be applied in a high-temperature concentrated alkali environment is a ferritic stainless steel having a high Cr content (see, for example, Non-Patent Documents 1 and 2). An example of such a stainless steel is SUS 447J1 (30Cr-3Mo). However, a stainless steel having a high Cr content of around 30 mass % is poorly available because it is difficult to manufacture, and even if it can be obtained, it has poor workability when it is used for the construction of plant equipment. Therefore, it undergoes marked deterioration in corrosion resistance, particularly in welds. In light of such problems, it is not being widely used.
Under relatively mild conditions in a high-temperature concentrated alkali environment, demands with respect to corrosion resistance are lower and it is possible to use a material having excellent workability. Under these conditions, certain duplex stainless steels are sometimes used. For example, Patent Document 1 states that SUS 329J4L is suitable. However, this material cannot be said to have sufficient corrosion resistance in a high-temperature concentrated alkali environment.